Rewinding Transformers

[Ed/KB1HYS]

I need two transformers for a project (a 4-250 final).  One filament (5volts/30 amps) and one plate transformer (3KV at .5 amp) both will use 120 volts in. 

I have lots of transformers of various sizes, but of course none that are what I need. So, having more time than money... (and not much time) I was considering winding my own. 

So for the questions: 

I have a large transformer that has a square core, with a winding on each opposite side of the core. It's the biggest I have, and I wanted to use it for the plate tranny. Any issues with using this type of core instead of the usual E type laminations??

The handbooks have formulas based on core size to determine the power handling abilities of a transformer.  Are these accurate enough?  or should I just go with the largest core size I can work with?

[WZ1M]

The iron you have will work just fine. Must be a dual primary, one on each leg of iron?
Gary...WZ1M

[WA1GFZ]

The transformer was wound to provide better isolation between primary and secondary. (Common mode rejection.) Core area is core area so it will work fine.

[WZ1M]

Alot of these "dual coil" transformers have 1/2 windings, both primary and secondary on each coil. Collins HV trannys are like this. Well, atleast the ones I have rewound.
Gary...WZ1M

[WA1GFZ]

Gary are you saying they are bifilar wound on the primary?
Also do they do anything special with the modulation transformer primary? fc

[WZ1M]

What I am saying, there are two bobbins per transformer. half the windings on one bobbin and the other half of the windings on the other bobbin. Now the two bobbins are slid onto each leg of iron, parrallel to each other. I have not seen this done in mod iron but do see it alot in the bigger HV iron. Some mod iron, as in Todds (KA1KAG) wind one half of the primary under the secondary and the other half of the primary over the secondary, creating a sandwich. Primary being the bread and the secondary being the strawberry jam.
Gary...WZ1M

[WA1GFZ]

Interesting. I thought  the reason for winding on the outside was isolation. I have a 50 volt transformer from a medical application like that. I suppose you get double window area using the outside posts and allows bigger wire for less copper loss.
I knew about the mod iron trick for primary winding. Some switcher power supply guys do that trick to reduce leakage inductance.
Good to have a guy in the biz on this board.

So the trick would be for Todd to connect the tube that makes positive modulation peaks to the primary winding closer to the core since it will have lower resistance and better coupling than the primary on the outside . It might be good to tell Todd which primary is closer to the core to see if it makes a difference.

[Jim, W5JO]

An interesting sidelight to this is someone posted on a related reflector that Peter Dahl has suspended Amateur products for the near future.  The poster said he received a call from them telling them to check back in about 3 months. 

According to the poster Dahl is covered up with commercial work and was tired of hearing amateurs calling and moaning about the delay.  So Dahl did the expedient thing.

[Ed/KB1HYS]

I don't know if the dual bobbin transformer is a parallel winding or not. I will have to look at it to see.  I had assumed that one winding was the primary and the other was the secondary...  (a johhny novice type mistake I guess.) 

So if it is two in parallel, how critical is it that the windings match? or is there a reasonable "close enough"??

[WA1GFZ]

Ed, if you parallel the windings they need to have the same number of turns on the core. Series across 220 won't be a problem if they are not equal turns.
Dahl too busy sounds like the perfect time for Gary to go big time and build better quality. I'm sure Dahl doesn't coat each layer as it is wound.

[wavebourn]

Did you consider microwave oven transformers?
They could work if to remove a magnetic short-cut

[Ed/KB1HYS]

I had thought about MW oven transformers.  But series paralleling two or more together to get the righ voltage would take up a lot of space and weight. 

Also I had thought that voltage regulation would suffer if I used MW transformers, instead of just one that was a bit larger than needed.

[WBear2GCR]

I think one issue with the core being a []  and a winding/bobbin on each " [ " will be that the magnetic path will be reduced as compared to an E-I configuration where the entire winding is on a section that is ~2x the outside loop (the visible part of the lams) or in ascii: [ | ]
the " | " has to be twice the thickness of each "[" to properly pass the flux.

On the "box" core the flux is limited to the path that is visible - so as long as the xfmr is designed with a primary on one leg and a secondary on the other, ur good to go. But if you put a primary and a secondary on each leg, the flux carrying capability simply needs to be taken into account, and rated or "derated" so that the core is not saturated. (I'm just assuming/thinking here that one could sit oversized windings on this core, that might exceed the core's flux capability)

But assuming the cores are big enough, then not an issue anyhow.

Depending on the geometry one may or may not get a bigger winding window...

Nothing prevents anyone from using an EI core with windings on the outside legs, and not the inside...?

A possible advantage for the two bobbin/two leg arrangement might be that the windings and so the DCR of the two bobbins will be exactly the same for each half. A disadvantage might be that the two halves are separated by time via the lams, and at high freqs the hysterisis may mess with the HF response and/or create phase shifts - not sure on this tho'. Assuming application as a mod iron.

But for power, seems to me that you could wind the primary on one leg and the secondary on the other, done. Look ma, almost no special insulating techniques needed!

The other thing is that you could wind identical windings on each leg, and parallel the primaries - this would have the nice advantage of making the primary wire thinner per bobbin than with a single bobbin. Or you could play with this and series the primaries making the voltage per primary lower... or the same sort of deal with the secondaries...

Thinking about this makes my head go like this ----> 

booga booga... 

                       _-_-WBear2GCR